The only time the Lib Dems mattered lately, they completely fucked it up. They ought to just join Labour.

On the other hand, if a Labour coalition including the Lib Dems is viable after the next election, and they insist on sticking around, for some reason, they ought to insist on voting reform being passed as a condition of coalition, not merely holding a referendum on it.

A switch to approval voting would prevent Conservatives from constantly taking advantage of vote splitting among Labour/Lib Dems/SNP/Greens/etc. It was foolish to try to get a concession on voting methods from the party that most benefits from plurality voting, but it might be possible to get it from Labour.